Field of the Invention
This invention relates to the field of sour water treatment and specifically to decontamination of hazardous sulfur compounds in oilfield produced water with a two-stage treatment process using an iron sponge and a stabilized sodium percarbonate solution.
Background of the Invention
Water returning from producing oilfields (e.g., oilfield produced water) may include hydrogen sulfide, mercaptans, and other hazardous sulfides such as disulfides. This oilfield produced water may originate from the native formation or may be water returned after injection for well stimulation activity. Typically, this oilfield produced water is returned to the formation or treated to reduce hazardous sulfides to a manageable level. Various regulatory agencies may also impose strict guidelines on allowable hazardous sulfides content prior to disposal. Methods have therefore been developed to treat such oilfield produced water.
Conventional water treatment methods include using solid or liquid hydrogen sulfide scavengers, oxidizing chemicals, or physical separation methods such as membrane technology or activated carbon. Solid scavengers such as zinc oxide or iron oxide sponges are typically used for complexing hydrogen sulfide in solid form. Liquid scavengers such as triazine or acrolein are typically used for complexing hydrogen sulfide in liquid form. Oxidizing chemicals typically include nitrites (i.e., sodium nitrite), hypochlorite, potassium or sodium permanganate. Oxidizing chemicals are conventionally used to convert hydrogen sulfide to more harmless forms of sulfur such as water-soluble thiosulfate or sulfate.
Drawbacks to such conventional treatment methods include that while liquid scavengers may be very cost-effective and may tie up hydrogen sulfide as water-soluble compounds that are discharged to wastewater treatment facilities, such facilities are typically inaccessible to oilfield applications. Direct discharge of the treated effluent may therefore be problematic. Further drawbacks include that while oxidizing chemicals may irreversibly convert hydrogen sulfide to harmless water soluble forms of sulfur that may be compatible with effluent discharge, the chemicals themselves may pose significant impact to the environment. Additional drawbacks include that permanganate forms reaction solids such as solid manganese dioxide as a reaction product with hydrogen sulfide. Drawbacks to permanganate also include that it costs more per pound and may not be permitted for disposal without removal of the reaction solids. Permanganate may also be dangerous as any residual hydrocarbon in the water may invoke an exothermic reaction, which is also a drawback to hypochlorite. In addition, while nitrite chemicals may be useful at eliminating hydrogen sulfide, heat may be required for activation (i.e., typically heat accelerates a rather slow oxidation of hydrogen sulfide), which may produce another hazardous chemical (e.g., ammonia) as a reaction product. Moreover, costs are typically higher for oxidizing chemicals than for liquid scavenger chemicals. Drawbacks to zinc salts and oxides include that they may be quite cost prohibitive and produce a solid sulfide-containing residue that may regenerate hydrogen sulfide upon acidification, which may also require a disposal cost. Triazines and acroleins, while typically suitable for wastewater treatment plants, may form compounds that make direct disposal of treated water problematic from a regulatory standpoint. Drawbacks to activated carbon include disposal, which may be a costly option for large amounts of hydrogen sulfide.
Iron sponges have been developed to treat gas streams for removal of hydrogen sulfide. Iron sponges, typically to a lesser extent, have been used for similar treatments in industrial water applications. SULFATREAT® (a registered trademark of M-I L.L.C.) markets a commercial product (SULFATREAT®HC) used for hydrogen sulfide remediation of non-potable water. This product may potentially remove most if not all of the hydrogen sulfide from water. Drawbacks include that over time, the efficiency may drop off as the bed is exhausted. Further drawbacks include that the product may be less efficient at removal of mercaptans.
Consequently, there is a need for an improved method for decontamination of hazardous sulfides in oilfield produced waters.